The invention concerns a process and a measuring device for the determination of carbon activity in furnace atmospheres or the like, in which the partial pressure ratio of the oxygen in the furnace gas with respect to a reference gas is determined with the aid of an electric potential supplied by oxygen ion conducting solid state cells.
It is known that in the heat treating of steel in certain cases, particularly in carburizing, the level of carbon (designated C level or C potential) of the furnace atmosphere is measured and regulated. The C level is a single valued function of the activity a.sub.C of carbon, i.e., its vapor pressure ratio with respect to the pure phase (graphite). The afore-mentioned processes are used to determine the activity a.sub.C of furnace atmospheres.
As in the similarly known older processes of dew point and CO.sub.2 measurements, the CO content of the furnace atmosphere must be taken into account in the above-mentioned process also, because the oxygen pressure depends on the P.sup.2 .sub.CO.sbsb.2 /P.sup.2 .sub.CO' ratio, but the a.sub.C activity is a function of the P.sup.2 .sub.CO /P.sub.CO.sbsb.2 ratio. Various methods have been applied to the solution of this problem. Thus, in the so-called carrier gas method a carrier gas with a known CO content is introduced in such volumes, that in case of a varying addition of carburizing gas the CO content of the furnace atmosphere will not be significantly altered.
The disadvantage is a relatively high consumption of energy and gas. It is further known to try to maintain a constant CO content in the furnace by means of a combination of certain alcohols, wherein as a trade-off the disadvantage is accepted that the choice of the carburizing agent is restricted. The CO content in the furnace has been continuously regulated with an additional measuring instrument and the C level corrected accordingly. The disadvantage of such a process is the high cost of investment and maintenance.
It is therefore the object of the present invention to provide a process of the afore-mentioned type whereby without the additional consumption of energy, with simple means the determination of the activity of carbon becomes possible. Simultaneously, a suitable measuring cell is provided whereby this object may be attained.